This invention relates to a developing unit used in electrophotography and more particularly to such a developing unit comprised of a magnet roller disposed opposite to a photosensitive drum, a doctor blade for controlling the amount of developer adsorbed on this magnet roller and a toner sensor for measuring toner concentration.
Most developing units for electrophotography use a two-component developing agent. In this case, it is necessary to keep a constant ratio between its toner and carrier concentrations and to uniformize the toner concentration in the developer so that the quality of the obtained prints can be maintained at a high level. If the toner concentration is too low, the print density may become insufficient and the excess carrier may fall from the developing tank. If the toner concentration is too high, on the other hand, the excess toner may scatter around inside the tank to affect the print quality.
The toner concentration in the developing tank is determined by indirectly measuring the amount of the carrier in unit volume by the change in magnetic permeability. A device used for such a measurement is called a toner sensor and is adapted to detect changes in the magnetic flux in the neighborhood of a coil caused by variations in the amount of the carrier containing iron. Such a sensor is not sufficiently sensitive and cannot measure the desired concentration accurately if it is not in contact with the developing agent. If it is buried inside the agent, however, it tends to obstruct its stirring motion.